International Mathematical Olympiad - Georg Mohr-Konkurrencen

Language:
English
Day: 2
Thursday, July 8, 2010
Problem 4. Let P be a point inside the triangle ABC. The lines AP , BP and CP intersect the
circumcircle Γ of triangle ABC again at the points K, L and M respectively. The tangent to Γ at C
intersects the line AB at S. Suppose that SC = SP . Prove that MK = ML.
Problem 5. In each of six boxes B 1 , B 2 , B 3 , B 4 , B 5 , B 6 there is initially one coin. There are two
types of operation allowed:
Type 1: Choose a nonempty box B j with 1 ≤ j ≤ 5. Remove one coin from B j and add two
coins to B j+1 .
Type 2: Choose a nonempty box B k with 1 ≤ k ≤ 4. Remove one coin from B k and exchange
the contents of (possibly empty) boxes B k+1 and B k+2 .
Determine whether there is a finite sequence of such operations that results in boxes B 1 , B 2 , B 3 , B 4 , B 5
being empty and box B 6 containing exactly 2010 20102010 coins. (Note that a bc = a (bc) .)
Problem 6. Let a 1 , a 2 , a 3 , . . . be a sequence of positive real numbers. Suppose that for some
positive integer s, we have
a n = max{a k + a n−k | 1 ≤ k ≤ n − 1}
for all n > s. Prove that there exist positive integers l and N, with l ≤ s and such that a n = a l +a n−l
for all n ≥ N.
Language: English
Time: 4 hours and 30 minutes
Each problem is worth 7 points